Yes, you are right, 90 degrees to the board is vertical, however Deboichet uses a scale that doesn't refer to angles or vertical at all; it can be a little confusing!

I don't know exactly how it came about, but basically back about 4-5 years when everyone started changing the rake of their fins to be more upright, Deboichet fins were about the only decent fins on the market and so people started saying "my fin is +2 or +4cm", which meant that their fin was 2cm more upright than the "original" fin was designed.

Eventually, this became a little more standardised (thank god!) and the popular Deboichet system used now is +11cm as vertical (although most fins are usually +6cm or +8cm) which is just a reflection of how far the fin is raked upright compared to the "original mould". Its a confusing scale as it doesn't reflect AT ALL how "upright" the fin is. It merely refers to the original rake on the fin mould (although this has somewhat changed over the years).

I prefer to talk in degrees where 0 degrees is vertical and 2.5 degrees is roughly a +8cm Deboichet fin (or 30mm back from vertical at the tip) as this is less confusing. The Deboichet rake system only applies to Deboichet fins!

I just mentioned the 30mm back from vertical just to show you that this is what is considered "+8cm" right now, as I'm sure if you have a Deboichet fin from 4-5 years back it might not be the same "original rake" as a fin that is made now.